Polypropylene crystallizes in a spherulitic morphology with inherently weak spherulite boundaries subject to fracture upon impact. The spherulite boundaries, which are critical to strength, tend to be weak causing premature failure under most loading conditions.
Polypropylene blends using ethylene as a toughening agent have been described, see for example "Morphology and Properties of Blends of Polypropylene with Ethylene-Propylene Rubber," Jang et al., Polymer Engineering and Science, V. 24, No. 8, Mid-June 1984 describing rubbers dispersed as small particles in the polypropylene matrix, "Polyesters to Polypeptide Synthesis," Encyclopedia of Polymer Science and Engineering, V. 12 pp 443-444 stating that polypropylene blends exhibit improved toughness, "Rubber-Toughening in Polypropylene," Jang et al., Journal of Applied Polymer Science, V. 30, (1985), pp 2485-2504 observing that the rubber particles were randomly dispersed in the polypropylene matrix, were engulfed by the growing matrix, and did not reside preferentially at the spherulite boundaries, "Crystalline Morphology of Polypropylene and Rubber-Modified Polypropylene," Journal of Applied Polymer Science, V. 29, pp 4377-4393 (1984) showing that the rubber phase is not preferentially pushed to the spherulite boundaries and the distribution of the rubber particles is random in the polypropylene matrix. "Propylene/ethylene-co-propylene blends: influence of molecular structure of EPR and composition on phase structure of isothermally crystallized samples," D'Orazio et al., Journal of Materials Science V 26, pp 4033-4047 (1991) discusses amorphous EPR minor component segregation mainly in spherical shaped domains distributed in intra and interspherulitic regions. However, no interspherulitic boundary strengthening was observed. Indeed, because the copolymers discussed are amorphous, they are unable to crystallize and hence cannot provide reinforced spherulite boundaries and interlamellar links, as in the present invention.
Several U.S. patents are also directed to propylene ethylene polymer compositions see for example U.S. Pat. Nos. 4,946,898 and 4,948,841 to Kasahara et al., U.S. Pat. No. 4,774,292 to Thiersault et al., and U.S. Pat. No. 4,395,519 to Minami et al.